In a tuning system for a receiver intended for tuning signals occupying identifiable frequency channels, such as a television receiver, it is desirable that the tuning system include provisions for automatically skipping certain channels. The skipped channels may be those not preferred by a user because of their generally unsatisfactory reception characteristics or because of their program content. Receivers providing this feature permit the viewer to select a "skip list" of nonpreferred channels and store this information in a separate memory or they may utilize special encoding to include the skip list information together with tuning information (such as the tuning voltage or channel number) in the same memory. Examples of prior art tuning systems are disclosed in U.S. Pat. No. 4,156,850, issued in the name of Billy W. Beyers, Jr. on May 29, 1979 and entitled "DISPLAY SYSTEM FOR FACILITATING THE SETUP OF A TUNING SYSTEM" and U.S. Pat. No. 4,158,816, issued in the name of Charles M. Wine on June 19, 1979 and entitled "MEMORY TYPE TUNING SYSTEM WITH PROVISIONS FOR SKIPPING NONPREFERRED TUNING POSITIONS".
It is desirable for the skip list to be retained in memory during the period when a mains power line operated receiver is switched off so as to be available whenever the receiver is again switched on. Generally, semiconductor memories are "volatile" and will only retain stored information so long as their operating voltage is maintained. Even the provision of a low power standby power supply derived from the mains power line will not safeguard the contents of a skip list stored in such a "volatile" memory since brief power outages are not uncommon and unplugging of the receiver power supply cord from the power outlet can also be expected. Loss of the skip list under these circumstances would be a considerable annoyance. A battery power supply may be incorporated in the receiver for maintaining the memory. However, batteries have a limited life and the need to keep replacing batteries in a mains power line operated receiver is inconvenient and the task is likely to be neglected. Also, manufacturers generally prefer to avoid the need for user access to any of the internal parts of a television receiver, as might be the case when battery replacement is required.
There exist semiconductor memories, known as non-volatile memories which retain their contents even after the power supply is removed. However, such memories tend to be more expensive and therefore are generally thought to be less desirable for use in, for example, mass produced television receivers.
It has been recognized that, in contrast with the periodic unavailability of a power supply in, for example, a television receiver, commonly used hand held remote control units for use with such receivers are themselves battery operated and that the remote control unit battery could be used to maintain a small power supply to a semiconductor memory, even at times when the remote control unit is not otherwise in use.
Prior skip list arrangements have generally stored the skip list in a random access memory, usually known as a RAM. A RAM permits any of its memory locations to be addressed at will through appropriate addressing circuitry, e.g., by way of "column" and "row" addresses. The present inventor has recognized that a RAM with enough memory locations for each of the channels (e.g., television broadcast channels 2-83 and cable distribution channels 2-99 in the United States) is of considerable complexity compared with the function of storing channels to be skipped. Moreover, the power requirement for maintaining the contents of a RAM is so high that if it were contained in a remote control unit, it would unacceptably reduce the battery service life.